Borrelia burgdorferi, the etiologic agent of Lyme disease, is maintained in an infectious cycle that involves the transmission of the bacterium from an Ixodes tick to a mammalian host. During transmission the spirochetes encounter numerous alterations in environmental stimuli, and respond to these alterations in the environment by modifying the expression of genes that may prove important in adaptation, infection and persistence. Many of the genes that are differentially expressed reside on the many of the linear (lp) and circular (cp) plasmids that can be isolated from this spirochete. To date, the function of many of these pH and temperature-regulated membrane proteins has not been determined. Of interest are several members of paralogous gene family (pgf) 54 that localize to plasmid lp54 that are up-regulated in vitro when spirochetes are exposed to a temperature and pH (37 degrees C and pH 7.0) that mimic the environment of the attached, fed tick or mammal. Likewise, many of these pgf 54 members are down regulated by laboratory conditions that mimic the free-living, unfed tick (23 degrees C and pH 8.0). Moreover, we have determined that sera from mice infected via tick-borne transmission recognize nineteen borrelial membrane proteins, two of which are members of pgf 54. In Specific Aim 1 we intend to assess using the proteins encoded by the pgf 54 members as diagnostic markers of early, early-disseminated, and late stage Lyme disease. In Specific Aim 2 we will determine the efficacy of using recombinant proteins encoded by pgf 54 as vaccine candidates. Furthermore, in Specific Aim 3 we will identify additional pH-regulated membrane proteins expressed by B. burgdorferi during tickborne infection of mice that may be of diagnostic interest.